The present invention relates to an electrostatic shield for an HVDC transmission component with at least two terminals, in particular for an HVDC transmission air-core coil.
Components for high-voltage d.c. (HVDC) transmission systems lie at high potential relative to earth, e.g. 500 to 800 kV, during operation and are thus exposed to continuous contamination and in some instances with heavy accumulations at some points, which is known as “electrostatic precipitation” or the “black spot” phenomenon. The reason for this is charge carriers that are constantly present in the atmosphere such as ions, ionisable or polarizable dust and dirt particles etc., which in the strong electrostatic field move between the component and its surrounding area to the surface of the component and are deposited there. The extent of electrostatic contamination is last but not least dependent on the amount of free charge carriers available in the surrounding area of the component, e.g. as they become detached from fences, supports in the vicinity etc. and migrate to the surface of the component. This not only generates the mentioned contamination, but also means charge build-up on the outer insulation of the component, which can lead to local discharges.
Various solutions for keeping electrostatic contamination away from HVDC transmission installations or at least reduce it have already been proposed. The use of a Faraday cage around the component or reinforcement of the outer insulation to prevent at least point-type discharges are mentioned as examples. However, these measures are relatively complex and costly to justify their use in practice, and therefore black spot phenomena have simply been accepted hitherto.